Heart pacemakers and other implantable medical devices are constructed having an outer housing within which the necessary electronic components are contained. Such outer housing must be formed of a material which is compatible with being placed inside the human body. It is also generally desirable to shield the electronics within such housing from external sources of electromagnetic interference (EMI). Titanium is often utilized to satisfy such dual requirements of biocompatibility and shielding.
At least one elongate lead (i.e., a wire) will generally extend from the electronics within such an outer housing to a desired location inside a user's body. While the outer housing may shield the internal electronics from direct EMI radiation, steps may also be taken to inhibit transmission of EMI along such elongated lead itself. For example, selected capacitive and/or inductive components may be mounted on a circuit board along with the other internal electronics in order to provide EMI filtering. Alternatively, a so-called feed-through style filter may be provided at the location where the elongated lead passes through the outer housing. Examples of such feed-through filter subject matter are shown and described in U.S. Pat. Nos. 5,999,398, and 6,459,935 B1, both assigned to the owner of the present subject matter, and the disclosures of which patents are fully incorporated herein by reference, for all purposes.
U.S. Pat. No. 5,999,398 to Makl et al. entitled “FEED-THROUGH FILTER ASSEMBLY HAVING VARISTOR AND CAPACITOR STRUCTURE” describes a feed-through filter assembly such as may be used in an implantable medical device. The assembly includes a conductive mounting element which may be hermetically sealed to an outer housing of the implantable medical device. In many embodiments, the conductive mounting element will be a conductive canister in which a feed-through filter structure is located. Alternatively, the conductive mounting element may include a suitable subplate structure. Because the filter structure exhibits varistor and capacitive characteristics, effective transient suppression and interference filtering is achieved in a single package. Secondary filtering may be provided downstream of the filter assembly for additional interference filtering at lower frequencies.
U.S. Pat. No. 6,459,935 B1 to Piersma entitled “INTEGRATED FILTER FEED-THRU” describes a multi-leaded, filter feed-thru assembly for implantable medical devices, such as heart pacemakers, defibrillators, and neurostimulators, which integrates both multi-element semiconductor devices and passive component devices, or multi-element combinations thereof, together with a discoidal capacitive filter device to provide filtration of electromagnetic interference is provided. The assembly additionally provides for the suppression of high voltage transients from defibrillation and electrocautery procedures, as well as providing additional circuit and network functions.
Although pacemaker signals are relatively low voltage, capacitors utilized in feed-through filtering arrangements must often be constructed to withstand relatively high voltage levels. This is to ensure that the capacitor does not become damaged if subjected to voltage transients, such as those which may be caused by defibrillation pulses. Particular concern is raised if some or all of the same leads used as output from certain implantable medical devices are also used as input leads for associated measurement technology because the energy exiting certain implantable devices may be higher than that entering the devices as input signals.
Additional exemplary information regarding filtering technology may be found in other patents, published patent applications, and publications, including Published U.S. Patent Applications 2005/0197677 A1, entitled “APPARATUS AND PROCESS FOR REDUCING THE SUSCEPTABILITY OF ACTIVE IMPLANTABLE MEDICAL DEVICES TO MEDICAL PROCEDURES SUCH AS MAGNETIC RESONANCE IMAGING” by Stevenson, published Sep. 8, 2005; and 2004/0263174 A1, entitled “MAGNETIC RESONANCE IMAGING INTERFERENCE IMMUNE DEVICE” by Gray et al., published Dec. 30, 2004; and U.S. Pat. No. 6,795,730 B2, entitled “MRI-RESISTANT IMPLANTABLE DEVICE” by Connelly et al.; U.S. Pat. No. 6,424,234 B1, entitled “ELECTROMAGNETIC INTERFERENCE (EMI) FILTER AND PROCESS FOR PROVIDING ELECTROMAGNETIC COMPATIBILITY OF AN ELECTRONIC DEVICE WHILE IN THE PRESENCE OF AN ELECTROMAGNETIC EMITTER OPERATING AT THE SAME FREQUENCY” by Stevenson; U.S. Pat. No. 5,896,267, entitled “SUBSTRATE MOUNTED FILTER FOR FEEDTHROUGH DEVICES” by Hittman et al.; and PUBLICATIONS: “Heart Devices May Be Safe for MRI Scan,” R& D Digest, October 2004; “New Pacemakers Prove MRI-Proof,” Spectrum Online, Oct. 21, 2004; and “Advances in MRI-Safe Technology: Interview with Michael L. Weiner, CEO, Biophan Technologies, Inc.,” EPlab Digest, July 2004.
While various implementations of feedthrough filter assemblies have been developed for use in association with medical implants and other input and/or output filtering applications, no design has emerged that generally encompasses all of the desired characteristics as hereafter presented in accordance with the subject technology.